Individuals of all ages, from sucking infants to persons of four score, are liable to it, but they are not equally subject to it. It is less likely to affect the very young, and the aged, than those of middle life. However, the very old are by no means exempt from the effects of the pernicious influence; and, with respect to the very young, some extremely curious statements have been made. It is said that persons have had Ague before they were born. We know that infants in the womb are liable to many diseases. One case of this kind is recorded by Dr. Russell in his "History of Aleppo." A woman had Tertian Ague, which attacked her, of course, every other day; but on the alternate days, when she was well and free, she felt the child shake; so that they both had the Tertian Ague, only their paroxysms happened on alternate days. Bark was prescribed for her; and it cured the little one first, and afterwards it cured the mother.

One probable reason why Ague more commonly affects middle-aged persons, than those at either extreme of life, is that the former are much more likely to be exposed to the primary exciting causes; and it is probably for the same reason that the disease is more frequently seen in men than in women.

The exciting cause of intermittent and remittent fevers is believed to consist in certain invisible effluvia or emanations from the surface of the earth, which were formerly called marsh miasmata, but to which it has become of late years fashionable to apply the foreign term malaria. The malaria is a specific material poison, producing specific effects upon the human body. These emanations which cause Ague have been called marsh miasmata because they are notoriously common in marshy places; but they arc not peculiar to marshy places. Of their physical or chemical qualities we really know nothing; we are made aware of their existence only by their noxious offects.

To the production of this poison a certain degree of heat seems necessary. It does not appear to exist within the Arctic Circle; nor does it manifest itself during the colder seasons of more temperate climates. In Great Britain it gives rise to intermittents, and principally to tertians. As we go South, in Spain, and along the shores of the Mediterranean, the remittent becomes the predominant form; and so also, in the Southern United States; and what is very instructive, remittents there contracted often improve into intermittents upon the removal of the patient to a colder climate.

In addition to heat, a certain degree of moisture is necessary to produce the Ague, and, as a country becomes drained and dry, most of the ague disappears. The Eastern coast of England, parts of Kent, Essex, Cambridgeshire, Norfolk, Lincolnshire, and the east riding of Yorkshire were notorious for the production of Ague; and James I, and Oliver Cromwell, both died of Ague, contracted in London. Both London, and much of the marshy land in the country has, of late years, been drained and brought into cultivation, and the consequence is that Ague is much less prevalent. In Canada, five and twenty or thirty years ago, Ague was very common, and the first settlers on new land used to suffer fearfully for the first few years; as the land becomes drained, however, the fever diminishes.

Agues, or aguish fevers, are endemic along every part of the low and level coast of Holland. In Italy, the Pontine marshes, rear Rome, have possessed for ages an infamous celebrity of the same kind. The whole of the district called the Maremna, stretching for about thirty leagues along the shores of the Mediterranean, and in some places ten or twelve leagues broad, is rendered dangerous, and almost uninhabitable, by the vast quantity of malaria annually evolved from its soil. In America, large districts are, for the same reason, prolific of disease. The late Bishop Heber, in his "Narrative of a Journey through the Upper Provinces of India," gives the following striking picture of the influence of the malaria in that part of the world. It seems to be alike pestiferous to man and beast.

"I asked Mr. Boulderson if it were true that the monkeys forsook these woods during the unhealthy months. He answered that not the monkeys only, but every thing which has the breath of life, instinctively deserts them from the beginning of April to October. The tigers go up to the hills; the antelopes and wild hogs make incursions into the cultivated plain; and those persons, such as dak-bearers, or military officers, who are obliged to traverse the forest in the intervening months, agree that not so much as a bird can be heard or seen in the frightful solitude. Yet, during the time of the heaviest rains, while the water falls in torrents, and the cloudy sky tends to prevent evaporation from the ground, the forest maybe passed with tolerable safety. It is in the extreme heat, and immediately after the rains have ceased, in May, the latter end of August, and the early part of September, that it is most deadly. In October the animals return. By the latter end of that month the wood-cutters and the cow-men again venture, though cautiously. From the middle of November to March troops pass and repass, and with common precaution no risk is usually apprehended."

It should be recollected that all malarious districts are much more dangerous at night than in the daytime. To sleep at night in such places, in the open air is almost to insure an attack of fever. It has repeatedly been observed among the crews of ships, when off a malarious coast, that the sailors could go on shore,in the day, to cut wood, or for other purposes, with impunity, while the men who remained on shore through the night guarding the water-casks, were many or all of them seized with the fever.

The marsh poison may also be conveyed by the wind. There is a striking anecdote given by Lancisi, shewing, on a small scale, the effect of the wind in carrying the malaria with it. Thirty ladies and gentlemen had sailed to the mouth of the Tiber on an excursion of pleasure. Suddenly the breeze shifted to the South, and began to blow over a marshy tract of land situated to windward of them. Twenty-nine of the thirty were immediately after attacked with Tertain Ague. And as the wind may thus transport the malaria to a distance, and thereby render a spot unhealthy which naturally might be salubrious, so also, it is often of service in clearing the poison from other places, and preventing its concentration.